Incorrect conditional type evaluation on transformed function type expressions

🔎 Search Terms

"conditional type", "function type", "extends", "evaluation", "inference"

🕗 Version & Regression Information

This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play?ts=4.7.4#code/PTAEEFQUwDwFygOwCYGdQCEBQcCeAHKCUAXlAAoA3AQwBsBXKALlFTgCcBLRAcwEpSAPlCUA9p2Q4CRDKQo0GzUAHIAFlFq1RygSWFiJWKYVAAxSGVMAecIKwh5dRizZdeu-eMnGip2ZasMOwcqJyUAFgAmDxEvIwc4dVAAY1EAW3x6BGQRMPRqdiJUjIKoHNT2QuTaPB9QAHkAaTlzaHgkNDNZAH5QDkZQFgAzOlQoAG57MFBQAD1u+LBEolEuHm46NvxC1FROUUR8wtBEUTg6xvqWm2FYBBR0ayDQXv6iYdGJqZm5he+AFVUnHQnAytCgaSQcGocH2iFAyFEUHQpzgbSBACNOGjlqBtqIMeC0nV6lcyABxG5te6dSnPV7sAYfWhjSYOGbzIx4EzWf7UjqPRDCMjkAB04oKPFQLH+7GohyGqzSAAUCtQ0qgrKq5ZCEOxNf9BIIYgAlKBwejsRD-aRWQ1c6SgSl8u4CsxC0hYGba9XmqD6u23doPUDcIb+0A+jX8kP0RAAa1OAHdEABtAC6XpeFHFosl0tAsvlqEV7BVao1WorqGNQlAZotVpthEDWZYiCglH9DpMRYVSqjBuFoAA3qBU81uKB41BcKIhoX0yxmq6QzO5wuM9n-hP04NC7uY501BotMps1F9zvGnuAL6TOqmeFkHMS9hSljy3AZmJxxOiFNxiAA

💻 Code

// A extends B
type A = (value: string) => void
type B = (value: 'hello') => void

type FA = F<A>
// (value: string) => void

type FB = F<B>
// (value: 42) => void

// the computed values are compared correclty
type OK = FA extends FB ? true : false;
// return false

// the original expressions are not
type KO = F<A> extends F<B> ? true : false;
// return true

// This implementation does not exhibit the problem
type OO = G<A> extends G<B> ? true : false;
// return false

type F<T extends Fn> = (...args: TransformParams<Parameters<T>>) => ReturnType<T>

type G<T extends Fn> =
  Parameters<T> extends infer Params extends unknown[]
  ? (...args: TransformParams<Params>) => ReturnType<T>
  : never

type TransformParams<T> = { [K in keyof T]: K extends keyof [] ? T[K] : T[K] extends 'hello' ? 42 : T[K] };

type Fn =  (...args: any[]) => unknown;

🙁 Actual behavior

The comparison of the expressions F<A> and F<B> is incorrect and is not equivalent to the comparison of their results.

🙂 Expected behavior

F<A> extends F<B> should be false because it should evaluate to ((value: string) => void) extends ((value: 42) => void), and string and 42 are unrelated.

Additional information about the issue

Interestingly, G, which should behave the same as F, does not exhibit the problem.